Multiple-strand helical springs have been known in the past and are believed particularly useful in torsion spring applications, such as in providing a return force for the throttle shaft of an automotive vehicle, because of their safety advantages. Should breakage occur in only one strand of wire of such a spring, the return force would be reduced but the spring would not usually become disabled or inoperative.
Such a spring, to be effective, should have the multiple wires that form its coiled portion unconnected to each other, so that each coiled wire of the spring is capable of functioning independently. However, all of the wires at least at one end of the spring should be permanently secured together.
It has been previously known to provide a double-wire torsion spring construction in which the two wires are integral portions of a single length of wire that, at the commencement of a manufacturing operation, is first folded upon itself at its midpoint to form a pair of parallel strands. The folded end is then fed into a torsion winding machine where the double strands are simultaneously coiled in a manner similar to that commonly used in connection with the forming of single-wire helical torsion springs.
It is believed evident that such a construction would be time-consuming and expensive to manufacture. For each spring so manufactured on a one-at-a-time basis, a double length of wire must first be cut from stock and then folded upon itself. After flattening the fold to reduce it in thickness or width, the folded end must then be fed into the torsion spring winding machine. Continuous operation is not possible because each piece of wire must be separately cut and folded before a winding step is commenced. In addition to these manufacturing complexities, a double-wire torsion spring of folded-wire construction has the disadvantages of inherent weakness and risk of fracture at the point of the fold. Furthermore, should permanent attachment of the wires at the opposite end of the spring be desired (especially in view of such weaknesses at the folded end of the spring), a separate, subsequent welding step would be necessary.
Accordingly, an important object of this invention lies in providing a highly efficient and effective method for making multiple-strand helical springs in which the strands are joined together at least at one end, and preferably at both ends, of the spring. No folding of wire is involved and, if desired, the spring may be formed of more than two strands similarly joined together at opposite ends of the spring but always with the strands of the coiled portions free to move or slide (to a limited extent) in relation to each other. The wire is cut only after each helical spring has been wound using a standard torsion machine. In a preferred embodiment of the method, such cutting or severing occurs simultaneously with a welding step performed at a measured distance (which constitutes the developed length of a spring) from the cutting point so that, during a successive cycle of operation, the parallel strands of wire will be advanced and the next cutting operation will occur at (or immediately adjacent to) the welded zone produced during the preceding cycle. Ideally, the cut is made through the welded zone so that the strands on opposite sides of the cut remain welded together; however, as brought out hereinafter, the cut may alternatively be made immediately in advance of the welded zone so that each spring, in its final form, has its strands joined together only at one end of that spring.
Briefly, the method of this invention involves the steps of arranging a plurality of wires in substantially parallel, contiguous relation. The wire may be unwound from separate supply spools and guided in a straight path towards the mandrel of a torsion coiling machine. Along that path, each wire may be considered to have integral first, second, and third sections of equal length with the first sections of the parallel wires having their leading end portions welded together in a first zone of permanent connection. Such wires are also welded together in a second zone of connection spaced from the first zone and located along a defined stretch of wire that includes the trailing end portions of the first sections and the leading end portions of the second sections. As the parallel wires are advanced, the first sections are progressively wound about the mandrel of the torsion machine. The coiling operation is momentarily interrupted when a selected number of coils have been formed and, during such interruption, all of the wires are transversely severed at a point along the defined stretch and are welded together, preferably simultaneously with the severing step, at a third zone of connection located behind the second zone at a distance equal to the distance between the first and second zones or a multiple thereof.
In a preferred embodiment, the severing step involves severing the wires at a point within the longitudinal limits of the second zone of connection so that, following that severing step, the trailing end portions of the wires of the first sections remain connected to each other and the leading end portions of the wires of the second sections also remain connected to each other. A single welding step therefore functions in permanently connecting the ends of the wires for each of two successively-wound torsion springs. Alternatively, the severing may occur at a point ahead of the second zone of connection so that the trailing end portions of the wires of the first sections become disconnected from each other while the leading end portions of the wires of the second sections remain permanently connected by the weld. In any case, the steps of arranging, winding, severing, and welding are continued in repeated cycles of operation with a completed coil spring being formed and released during each cycle of the progressive manufacturing procedure.
The result of such a method is a helical torsion spring having two or more spring wires disposed in parallel, contiguous relation and arranged in a helix composed of a series of coaxial coils. The wires are unconnected and capable of limited independent movement along nearly the full length of the spring but all of the wires are permanently joined by welding at least at one end of the spring. In a preferred form of the invention, the opposite ends of the spring have all of the wires welded together at those points.
Other features, advantages, and objects of the invention will become apparent from the specification and drawings.